The invention relates to a directly mixing synchronous receiver having an RF input and a first signal path which is coupled thereto, incorporating a first synchronous demodulator and a first low-pass filter, and having a carrier regeneration circuit comprising a first phase-locked loop incorporating in a loop configuration a first phase detector which is coupled to the first signal path, a first loop filter and a first voltage-controlled tuning oscillator an output of which is coupled to the first phase detector, on the one hand, and to the first synchronous demodulator via a phase shift circuit, on the other hand, for a direct demodulation of an RF reception signal to the frequency baseband.
A directly mixing synchronous receiver of this type is known per se from British Patent Application No. 2,130,826.
In the first synchronous demodulator of a directly mixing synchronous receiver of the type described above, the modulated carrier of a desired RF reception signal is mixed with a tunable local mixing carrier which is tuned to the frequency of the desired RF reception carrier. The modulation signal modulated on this desired RF reception carrier is thereby directly demodulated to the baseband. The baseband modulation signal thus obtained is selected in the first low-pass filter.
For a correct direct demodulation of the modulation signal, the local mixing carrier should not only be equal in frequency to the RF reception carrier of a desired RF reception signal within the tuning range of the receiver, but it should also be accurately in phase or in anti-phase therewith. The local mixing carrier is generally derived from the output signal of the first voltage-controlled tuning oscillator of the first phase-locked loop.
The first phase-locked loop provides an automatic frequency control or an automatic fine tuning so that an accurate tuning of the oscillator signal applied to the first phase detector, to the desired RF reception carrier is possible, even in the case of a comparatively inaccurate tuning operation. In the phase-locked condition of this loop, the two signals applied to the first phase detector, that is to say the desired RF reception carrier and the last-mentioned oscillator signal, are mutually equal in frequency and differ by 90.degree. in phase with respect to each other. Due to this phase-quadrature relation with the RF reception carrier, the oscillator signal is not suitable for use as a local mixing carrier for the first synchronous demodulator.
To obtain a local mixing carrier which is phase-shifted by 90.degree. with respect to the oscillator signal in the loop, the first voltage-controlled tuning oscillator of the known directly mixing synchronous receiver first supplies an auxiliary oscillator signal whose frequency, upon correct tuning to a desired RF reception carrier, is twice the frequency of this RF reception carrier. From this auxiliary oscillator signal said oscillator signal for the first phase detector is obtained, on the one hand, by halving the frequency of the auxiliary oscillator signal and, on the other hand, a local mixing carrier differing by 90.degree. in phase with respect to the oscillator signal being derived by a 180.degree. phase shift of said auxiliary oscillator signal in the phase shift circuit, followed by halving its frequency.
In the known directly mixing synchronous receiver the tuning range of the first voltage-controlled tuning oscillator should therefore be twice as high as the RF reception range. For use as a TV-receiver, the tuning range of the tuning oscillator should cover a frequency range of approximately 2.times.50 MHz to 2.times.960 MHz. Such tuning oscillators are costly and complicated and due to the high oscillator frequencies they introduce unwanted parasitic effects such as crosstalk which disturb a good signal processing.
To avoid such high oscillator frequencies it is known per se to provide the tuning oscillator with a transistor output stage having a capacitively loaded emitter output and a collector output so that the oscillator signals available at the two outputs are mutually in phase quadrature. However, in practice the phase quadrature relation obtained in this way is found to be frequency-dependent and particularly at high tuning frequencies this phase quadrature relation is lost.